A Passive Optical Network (PON) is a technology for the access network of a communications system. A PON typically has a central office called an Optical Line Termination (OLT) which interfaces with a metro or carrier network and an arrangement of optical fibres and splitters which connect the OLT with multiple Optical Network Units (ONU) or Optical Network Termination (ONT) units. In a Fibre To The Home (FTTH) system an ONU is located at a subscriber premises while in a Fibre To The Curb (FTTC) system an ONU is located at a roadside cabinet.
More recently, Wavelength Division Multiplexed Passive Optical Networks (WDM PON) have been proposed. A WDM PON supports multiple wavelength channels. A separate wavelength can be allocated for communication between the Optical Network Unit (OLT) and each ONU in the PON. The potential advantages of using WDM techniques in fibre access networks include increased capacity, service agnostic end-to-end connectivity, bit rate transparency, easy channel upgrade, long distance reach, simplified network operation and maintenance.
There are three main classes of WDM PON architectures: (i) a WDM PON based on tunable lasers at the ONU; (ii) a WDM PON based on a remotely seeded ONU; and (iii) a WDM PON based on downstream remodulation.
In the first class of system, the optical carrier that is used for upstream communication (ONU to OLT) is locally generated at the ONU, using a tunable laser. The use of tunable lasers allows the longest link distance (reach) due to a simple system architecture, low optical path loss and absence of reflections induced penalties. Drawbacks are the cost of providing a laser at each ONU, the reduced capacity (two different wavelengths are needed for the upstream and downstream channels) and the need of a handshake protocol between OLT and ONU for the automated adjustment of the upstream optical carrier frequency.
In the second class of system, the optical carriers are remotely generated, typically at the OLT, and then distributed to the ONUs, where a device such as a Reflective Semiconductor Optical Amplifier (RSOA), Reflective Electro-absorption Modulator (REAM) or injection locked Fabry-Pérot Laser Diode (FPLD) can be used to modulate the upstream signal. The advantage of the second class of systems is the lower cost of a RSOA, REAM or FPLD compared to that of a tunable laser. However, the achievable distance is shorter because the upstream optical carrier experiences twice the link attenuation and the reflections at the connectors or within the fibre (Rayleigh backscattering) couple unmodulated and modulated signal travelling into opposite direction along the same fibre, giving rise to a cross-talk penalty. This class of system is shown in the paper “High-speed WDM-PON using CW injection-locked Fabry-Pérot laser diodes”, Xu et al, Optics Express Vol. 15, No. 6, March 2007, pp. 2953-2962.
In the third class of system, the downstream signal is a continuous envelope modulated signal which is tapped before the ONU receiver and remodulated by the upstream signal. The third class of system experiences the same issues, with the additional drawback that the remodulating device used at the ONU (e.g. a RSOA) is non-ideal and can cause further interference on the upstream signal, due to residual downstream modulation. A known method is to use a constant envelope modulation format in downstream (FSK, DPSK, etc.) so that no amplitude fluctuations affect the amplitude modulation used instead for the upstream. In such a kind of system, means for phase modulation-to-amplitude modulation conversion are needed at the ONU in order to correctly receive the downstream signal because a photodiode is sensitive only to amplitude variations and not to phase variations. The main advantage compared to the other two classes is the doubled capacity, because exactly the same wavelength is used in downstream and upstream. This class of system is shown in the paper “An Optical Network Unit for WDM Access Networks with Downstream DPSK and Upstream Remodulated OOK Data using Injection-locked FP Laser”, Hung et al, IEEE Photonics Technology Letters, Vol. 15, No. 10, October 2003.
H-H. Lin et al., “WDM-PON Systems Using Cross-Remodulation to Double Network Capacity with Reduced Rayleigh Scattering Effects”, OFC 2008, paper OTuH6 describes a cross-PON concept. A first OLT transmits a downstream signal which is used to transmit data to a first group of ONUs and also serves as a seed signal for a second group of ONUs. A second OLT transmits a downstream signal which is used to transmit data to the second group of ONUs and also serves as a seed signal for the first group of ONUs. Demodulation of the downstream signal is performed individually at each ONU.